I highly recommend Milltek exhausts, an Audi technician told me that it's the nicest sounding exhaust for these cars.
"On the highway, sport mode is absolutely amazing; words can't accurately replicate or describe the sound. We couldn't be more pleased with the performance, look, and sound of the Milltek valved cat-back exhaust system on our RS 4."You can read more in our 'Milltek in the Press' section or view the article, including dyno tests and photos, in full on the Fourtitude website.